This project is based upon our original observation that sulfhydryl (SH) compounds potentiate the inhibitory effect of selenite on the activity of DNA and RNA polymerases. We originally proposed to investigate the mechanism of this effect, and its relevance to the inhibition by selenite of DNA and RNA synthesis in the cell. During the past project period we have shown that inhibition of the polymerases results form the reaction of selenite with the SH compounds to form selenotrisulfides. In the case of RNA polymerase II, inhibition is limited to the initiation stage of the reaction. We have also found that upon exposure of the polymerases to selenotrisulfidesa Se-protein adduct is formed. During the next project period we will investigate the structure of the adduct and the specificity of its formation at different stages of the RNA polymerase II reaction. During the past project period we examined the effect of depleting cells of SH compounds on the inhibitory effect of selenite on nucleic acid synthesis. The result have provided direct evidence for the involvement of cellular SH compounds in the inhibition by selenite of DNA and RNA synthesis in living cells. Our results have dictated, however, that the predominant cellular SH compound, glutathione, is not involved in this effect. Dung the next project period we will examine the intracellular fate of exogenous selenite in order to determine which cellular SH compounds are involved. We will also examine the possible involvement of SH compounds in the inhibition by selenite of cell proliferation and cell transformation.